Question 8.2

I can't find anything in the Gnus manual about X
(e.g., attachments, PGP, MIME...), is it not documented?

Answer

There's not only the Gnus manual but also the manuals for message,
emacs-mime, sieve, EasyPG Assistant, and pgg. Those packages are
distributed with Gnus and used by Gnus but aren't really part of core
Gnus, so they are documented in different info files, you should have
a look in those manuals, too.